The vertebral spine is the axis of the skeleton upon which all of the body parts “hang.” In humans, the normal spine has seven cervical, twelve thoracic and five lumbar segments. The lumbar segments sit upon a sacrum, which then attaches to a pelvis, in turn supported by hip and leg bones. The bony vertebral bodies of the spine are separated by intervertebral discs, which act as joints, but allow known degrees of flexion, extension, lateral bending and axial rotation.
Each intervertebral disc serves as a mechanical cushion between the vertebral bones, permitting controlled motions within vertebral segments of the axial skeleton. For example, FIG. 4 illustrates a healthy intervertebral disc 30 and adjacent vertebrae 32. A spinal nerve 34 extends along the spine posteriorly thereof.
The normal disc is a unique, mixed structure, comprised of three component tissues: The nucleus pulposus (“nucleus”), the annulus fibrosus (“annulus”), and two opposing vertebral end plates. The two vertebral end plates are each composed of thin cartilage overlying a thin layer of hard, cortical bone which attaches to the spongy, richly vascular, cancellous bone of the vertebral body. The end plates thus serve to attach adjacent vertebrae to the disc. In other words, a transitional zone is created by the end plates between the malleable disc and the bony vertebrae.
The annulus of the disc is a tough, outer fibrous ring that binds together adjacent vertebrae. This fibrous portion is generally about 10 to 15 mm in height and about 15 to 20 mm in thickness, although in diseased discs these dimensions can be diminished. The fibers of the annulus consist of 15 to 20 overlapping multiple plies, and are inserted into the superior and inferior vertebral bodies at roughly a 30 degree angle in both directions. This configuration particularly resists torsion, as about half of the angulated fibers will tighten when the vertebrae rotate in either direction, relative to each other. The laminated plies are less firmly attached to each other.
Immersed within the annulus, within the intervertebral disc space, is the nucleus pulposus. The annulus and opposing end plates maintain a relative position of the nucleus in what can be defined as a nucleus cavity. The healthy nucleus is largely a gel-like substance having high water content, and similar to air in a tire, serves to keep the annulus tight yet flexible. The nucleus-gel moves slightly within the annulus when force is exerted on the adjacent vertebrae with bending, lifting, etc.
Under certain circumstances, an annulus defect (or anulotomy) can arise that requires surgical attention. These annulus defects can be naturally occurring, surgically created, or both. A naturally occurring annulus defect is typically the result of trauma or a disease process, and can lead to a disc herniation. FIG. 5 illustrates a herniated disc 36. A disc herniation occurs when the annulus fibers are weakened or torn and the inner tissue of the nucleus becomes permanently bulged, distended, or extruded out of its normal, internal annular confines. The mass of a herniated or “slipped” nucleus 38 can compress a spinal nerve 40, resulting in leg pain, loss of muscle control, or even paralysis.
Where the naturally occurring annulus defect is relatively minor and/or little or no nucleus tissue has escaped from the nucleus cavity, satisfactory healing of the annulus can be achieved by immobilizing the patient for an extended period of time. However, many patients require surgery (microdiscectomy) to remove the herniated portion of the disc. FIG. 6 illustrates a disc from which a portion has been removed through a microdiscectomy procedure. After the traditional microdiscectomy, loss of disc space height can also occur because degenerated disc nucleus is removed as part of the surgical procedure. Loss of disc space height can also be a source of continued or new lumbar spine generated pain.
Further, a more problematic annulus defect concern arises in the realm of anulotomies encountered as part of a surgical procedure performed on the disc space. Alternatively, with discal degeneration, the nucleus loses its water binding ability and deflates, as though the air had been let out of a tire. Subsequently, the height of the nucleus decreases, causing the annulus to buckle in areas where the laminated plies are loosely bonded. As these overlapping laminated plies of the annulus begin to buckle and separate, either circumferential or radial annular tears can occur, which can contribute to persistent and disabling back pain. Adjacent, ancillary spinal facet joints will also be forced into an overriding position, which can create additional back pain.
In many cases, to alleviate pain from degenerated or herniated discs, the nucleus is removed and the two adjacent vertebrae surgically fused together. While this treatment can alleviate the pain, all discal motion is lost in the fused segment. Ultimately, this procedure places greater stress on the discs adjacent the fused segment as they compensate for the lack of motion, perhaps leading to premature degeneration of those adjacent discs.